Dwarf planet Makemake an airless world of ice and rock

Dwarf planet is a mishmash of features shared with Pluto, Eris, and Haumea

Artist's impression of Makemake, a dwarf planet roughly 2/3 the diameter of Pluto.

ESO/L. Calçada/Nick Risinger (skysurvey.org)

The outer reaches of the Solar System are home to many small bodies, of which Pluto is the most famous. Due to their distance from Earth and relatively small sizes, these trans-Neptunian objects are somewhat difficult to study: even our most powerful telescopes can't image their surfaces in any detail, leaving astronomers either waiting impatiently for the New Horizons space probe to reach Pluto or resorting to more indirect methods. One such method is occultation, where one of these bodies briefly blocks (or occults) the light of a star.

A group of astronomers used 7 different telescopes in South America to track the occultation of a star by the dwarf planet Makemake (pronounced MAHkayMAHkay) and measured many of its properties for the first time. They concluded Makemake is noticeably non-spherical and may consist of two distinct types of terrain to explain the surface brightness.

When a Solar System object passes directly in front of a star, astronomers can measure both how much light that object blocks and how quickly that blocking occurs. And it's possible to combine data obtained by different telescopes in different locations on Earth (or ideally between multiple occultation events). Combined, these make it possible to reconstruct both the size—via the average length of the light blocking—and the shape of the body. In the absence of direct imaging, this is often the best we can do for a small, distant body.

Dwarf planets

The "official" dwarf planets designated by the International Astronomical Union are Ceres (the largest body in the Asteroid Belt), Eris, Haumea, Makemake, and Pluto (the four biggest objects in the region beyond Neptune's orbit). Of those, Eris is the most massive, though it and Pluto are nearly the same size. This hints at some significant variation in the composition of the trans-Neptunian objects, as does the contrast between Pluto's thin atmosphere and Eris' lack thereof. Makemake is roughly 2/3 the diameter of Pluto

The author of this article would like it to be known that his favorite trans-Neptunian object is Quaoar, which hasn't yet been recognized as a dwarf planet by the IAU.

Additionally, the abruptness of the occultation can reveal the presence of an atmosphere. If the eclipse occurs gradually, with a gentle decrease in the light from the background star, then the foreground body has an atmosphere. (What astronomers are observing is light passing through ever-thicker layers of gas until the surface of the body passes in front of the star.) In this case, researchers can measure the extent and density of the atmosphere, and spectral analysis may be able to determine its chemical composition as well.

On the other hand, if the star vanishes like switching off a light, then the foreground body has little or no atmosphere. The rocky or icy edge of the object blocks all the star's light effectively in an instant.

Makemake potentially occults three stars in a typical year, though not all of these are useful, due to the faintness of the background star. The current study involved an occultation visible from South America on April 23, 2011. Just as eclipses may be partial or total, the "shadow" of Makemake passed over a swath of the continent, allowing telescopes in various locations to measure the passage of the star behind different parts of the dwarf planet. The researchers tried to obtain data from 16 telescopes, but only 7 of those returned successful measurements.

Each telescope saw a clear, sharp drop in the background star's light, a strong indicator that Makemake has no substantial atmosphere. The astronomers also failed to see a second occultation that would reveal the presence of a moon, though they can't rule out a satellite smaller than 200 kilometers in diameter. (Previous observations have not seen a moon either.)

The different telescopes also saw the occultation last noticeably different lengths of time. The researchers modeled several possibilities and concluded the most likely one is that Makemake is not spherical. While it's nowhere nearly as elongated as the egg-shaped Haumea, if this model is correct, it is roughly 5 percent larger along one direction than another, making an ellipsoid shape roughly 1430km along one axis and 1500km along another. (Obviously this picture isn't complete, since we lack the data on the third dimension that was along our line of sight.)

The final bit of intriguing information the researchers collected involved Makemake's albedo, the fraction of light it reflects back into space. (Albedo is relative to the wavelength of light: an object may be highly reflective in visible light, but less so in other colors.) A completely reflective surface has an albedo of 1, while a completely absorbing surface would have an albedo of 0.

(Eris is highly reflective, with a visible-light albedo of 0.96; Pluto is much darker with 0.52 albedo. By contrast, the Moon's albedo is 0.12, about the same as a classroom blackboard.)

Makemake's average albedo is 0.77, but the observations revealed significant variation. The researchers argued these fluctuations could be reconciled if the dwarf planet has two types of terrain: highly reflective regions resembling Eris mixed with darker surface features like Pluto.

The astronomers suggested that this might be related to Makemake's lack of atmosphere. Eris' extremely bright surface is thought to have been created when its atmosphere froze and made a shiny frost across the globe. This same effect may have been responsible for the bright terrain on Makemake.

In this scheme, the atmosphere partially froze, leaving some traces over the darker, unfrosted regions that were too thin to be spotted in the occultation event. This model would place Makemake in a transitional region between Pluto, with its dark surface and measurable atmosphere, and Eris, with its high albedo but no air. With the limited data and few objects observed so far, it's hard to say whether this model is promising or not, but the suggestion is nevertheless intriguing.

Is it possible the ellipsoid shape might be responsible for the differences in atmosphere across the planet's surface? It seems likely that points farther from the center of the planet would have no or at least less atmosphere to freeze, so it would create the expected irregularity. Just an idea.

"(Eris is highly reflective, with a visible-light albedo of 0.96; Pluto is much darker with 0.52 albedo. By contrast, the Moon's albedo is 0.12, about the same as a classroom blackboard.)"

The Moon is as dark as a blackboard, really? It's funny how it looks so white and grey when I look up at it, and when watching films of the Apollo crew.

I was surprised by the Moon's albedo value as well, but then again, the Sun is really, really, astoundingly bright, and the Moon is only 8 light minutes away from it, which is probably why it appears as it does.

EDIT: When you think about it, it's probably a good thing that the Moon's albedo is so low, otherwise you might go blind staring at it.

"(Eris is highly reflective, with a visible-light albedo of 0.96; Pluto is much darker with 0.52 albedo. By contrast, the Moon's albedo is 0.12, about the same as a classroom blackboard.)"

The Moon is as dark as a blackboard, really? It's funny how it looks so white and grey when I look up at it, and when watching films of the Apollo crew.

I was surprised by the Moon's albedo value as well, but then again, the Sun is really, really, astoundingly bright, and the Moon is only 8 light minutes away from it, which is probably why it appears as it does.

Luna (Earth's moon) is also quite large for a satellite. IIRC Charon and Pluto are the only pair with a greater satellite:parent ratio.

Also, for those who don't know, our moon roughly has two regions: the lighter, rocky highlands, and the darker ancient basaltic sheets called maria ("seas"; singular mare). Almost all of the maria are on the side facing Earth, and the largest has a special name: Oceanus Procellarum (ocean of storms).

"(Eris is highly reflective, with a visible-light albedo of 0.96; Pluto is much darker with 0.52 albedo. By contrast, the Moon's albedo is 0.12, about the same as a classroom blackboard.)"

The Moon is as dark as a blackboard, really? It's funny how it looks so white and grey when I look up at it, and when watching films of the Apollo crew.

I don't think the albedo of celestial objects is very useful in terms of thinking about how things look, you know, to your eyes. That said, think of it this way: A blackboard looks kinda black and our moon looks kinda white, but they are both crappy mirrors.

"(Eris is highly reflective, with a visible-light albedo of 0.96; Pluto is much darker with 0.52 albedo. By contrast, the Moon's albedo is 0.12, about the same as a classroom blackboard.)"

The Moon is as dark as a blackboard, really? It's funny how it looks so white and grey when I look up at it, and when watching films of the Apollo crew.

Sunlight is surprisingly bright, even here on Earth and about 30% brighter at the top of the atmosphere (ie on the surface of the Moon too). The awesome dynamic range of our eyes really fools our intuitive sense of how bright it is outside at noon compared to what we would normally think was a very-lit room (though the commonly bitter experience of photographing mixtures of sun and shade points to this).

So imagine a one square metre blackboard with about 15 100W incandescent lightbulbs backed by a good mirror placed close to it. How black do you think it will look? Now cut a hole in the middle of the board and have a large dark room behind it so that very little of the light passing through or around the board will reflect back - how bright does the board look compared to that blackness?

Take a look at some photos of moon rocks - they often look like young lava, because in effect they are (basalts without biological processes rotting them to nice soils), eg:

In this scheme, the atmosphere partially froze, leaving some traces over the darker, unfrosted regions that were too thin to be spotted in the occultation event. This model would place Makemake in a transitional region between Pluto, with its dark surface and measurable atmosphere, and Eris, with its high albedo but no air.

Never mind the transition region, I honestly can't envision a spotty frozen atmosphere, even less one which is partially frozen. It screams finetuning several times in series.

Wouldn't the ubiquitous factor of surface albedo variation, impactors be the more likely mechanism?

It's amazing how much information you can get just by watching a star get blinked out like this.-the size-the orbital speed-rough shape-if there is an atmosphere (and if there is, it's composition)-it's albedoIf you have enough telescopes watching you can map the profile with surprising accuracy

I've always wondered why we can't use Hubble or another "really big" telescope to see in more detail these planets or moons?

When Hubble can see in such detail at such distances - can it not focus closer? (like hyperopia?) or is it that these objects move too quickly for Hubble to track?

Just curious. . .

A telescope which looks at astronomical objects have an effectively infinite depth of field unless I am mistaken, the light rays are always nearly parallel. So it shouldn't matter what actual distance they are.

The problem is instead one of resolution. If you take Hubble's resolution and check the size of what it can resolve at the distance of the Moon, it can resolve about a football field size of smallest detail.

I've always wondered why we can't use Hubble or another "really big" telescope to see in more detail these planets or moons?

When Hubble can see in such detail at such distances - can it not focus closer? (like hyperopia?) or is it that these objects move too quickly for Hubble to track?

Just curious. . .

It's too far away. I did a little math, and calculated that Hubble has a maximum angular resolution (for visible light) about 2 orders of magnitude greater than the apparent size of Makemake, so unfortunately no, you cannot (assuming I did my math correctly).

Quick primer: thanks to the fact that light is a wave, if two distant points are too close together, they cannot be distinguished by a telescope, because the waves from the two points will blend together. This angle depends on the size of the telescope and the frequency of the light (used 500nm for visible light... fun fact, when doing conversions Wolfram straight up told me I got nearly the right answer... I swear that thing is telepathic sometimes).

Anyways, Makemake is too small to be distinguished by Hubble at this distance.

Why after sending that probe all the way out to Pluto didn't they have the thing come in and orbit as part of the plan. That is probably where the aliens are hiding with Elvis is my hunch and 'they' just don't want us to know!

"(Eris is highly reflective, with a visible-light albedo of 0.96; Pluto is much darker with 0.52 albedo. By contrast, the Moon's albedo is 0.12, about the same as a classroom blackboard.)"

The Moon is as dark as a blackboard, really? It's funny how it looks so white and grey when I look up at it, and when watching films of the Apollo crew.

If you watch those same videos, consider that sitting in full sunlight (with zero atmosphere) it still looks dark and murky. The Moon looks white and glowing to us because it's gigantic and so close to us so the amount of reflected sunlight the Earth receives from it can be just bright enough to light up our nights (if it had a much higher albedo it would be an incredibly bright sight!). That doesn't change the fact that the Moon is largely a dark grey-ish mass with extremely poor reflectivity.

Why after sending that probe all the way out to Pluto didn't they have the thing come in and orbit as part of the plan. That is probably where the aliens are hiding with Elvis is my hunch and 'they' just don't want us to know!

New Horizons will be a fly by mission because it's time sensitive. It is now Winter on Pluto, it's heading out further from the Sun (it doesn't have a circular orbit), and getting there only gets harder with each passing year. The fly by was the fastest route to Pluto to get a probe there so it could study Pluto's atmosphere before it was entirely frozen out onto the surface. New Horizons actually has the fastest launch velocity of any other probe from Earth to date. Speed was essential. The probe was a bit like Voyager, sent on a solar escape trajectory.

If you wanted to orbit Pluto, you'd need a much lower velocity, gravity assists or even aerobraking when necessary to ensure you hit Pluto at just the right speed to be captured by its gravity. It would have taken far too long.

Getting to examine Pluto's atmosphere and early Winter conditions was considered far more important that a permanent stay. Good call. With a bit of luck the probe can be rerouted to visit other objects in that deep region of the Solar System.